Autostereoscopic 3D displays use a parallax barrier or lenticular lenses to generate multiple views. For a reflective or transflective display, where ambient light is used to illuminate the pixels of the display, such an optical element introduces an inhomogeneous pixel illumination. This effect is especially visible under (semi)-directional lighting conditions such as, but not limited to, sunlight or indoor spot lights. As a result of the inhomogeneous pixel illumination, certain views will be less bright or even completely absent.
Reflective displays have a paper-like appearance that is generally believed to look more natural than emissive displays. The main difference between emissive and reflective displays is the employed light source. In emissive displays either a combination of backlight with liquid crystal pixels or emissive pixels themselves are used to generate images. Reflective displays, on the other hand, selectively reflect ambient light in order to display their content. As a consequence, where the visibility of emissive displays generally decreases under ambient light, reflective displays excel under these natural lighting conditions. Furthermore, as reflective displays do not employ their own light source they consume only a low amount of power, giving them a strong advantage especially in mobile devices where they facilitate extraordinary long battery lives, for example of up to several weeks.
A combination of both types of displays is a transflective display, which can either operate in reflective mode without using backlight, or can adjust to the dark conditions switching to the transmissive mode of pixel operation with a backlight. In this case the display is composed of pixels of transmissive and reflective types.
Depending on the type of the reflective display (electrophoretic, electrowetting, stacked electrowetting etc.) the pixels can reflect the incoming light either dominantly by themselves, can be switched between reflective and transmissive modes, or change their transmittance. In the last two cases the light after propagation through the pixels gets reflected from a back reflector inside the display. This reflector can be a specular reflecting mirror, surface structured mirror or diffuse reflecting component.
In a 3D display, by placing a parallax barrier or lenticular lens on top of a 2D display, an autostereoscopic 3D display can be created that generates a plurality of views in space. The user is provided with the illusion of depth in the image by observing different views with the left and the right eye. By creating more than two views, the system can even accommodate, for a limited motion, parallax thereby enhancing the depth perception.
Existing 3D displays mainly employ emissive displays. The use of parallax barrier or lenticular lenses for reflective displays is highly non-trivial and intrinsically distinct from emissive displays due to the difference in illumination. This difference arises from the fact that the ambient light has to travel twice through the same optical element; both when illuminating the pixels and when the light is reflected from the pixels towards the user.
FIG. 1 shows this problem graphically. The display is a reflective display covered with a lenticular foil 2. FIG. 1(a) shows the display under directional ambient illumination 1. The ambient light is focussed onto the reflective layer, thereby illuminating only a small subset 4 of the available pixels 3. FIG. 1(b) shows that the subset of illuminated pixels can generate only a limited amount of views 5 (FIG. 1(b) ignores the views from the same pixels in other viewing cones).
This arises because the illumination light is inhomogeneously distributed over the display in the direction of the lenticular lens pitch. As a result, certain pixels, responsible for the corresponding views, will be less illuminated or not illuminated at all. The corresponding views will therefore be less bright and will be perceived by a user as dark bands. This effect will be especially pronounced under (semi)-directional ambient illumination conditions, such as direct sun light or indoor overhead lights.
To estimate the influence of directionality of illumination, typical device parameters for autostereoscopic 3D displays can be considered. The typical ratio of lens pitch to the distance between the pixel plane and the lens apex of about ⅙ to ⅛ results in an angular width of one viewing cone of 7 degrees to 9.5 degrees. For office illumination with overhead lights (lamp size 10 cm to 20 cm at 3 m height ceiling, such as downlights or TL-tubes) the typical angular spread of illumination rays at the device would be 2 degrees to 4 degrees, which means that only a small proportion of the views (⅕ to ½) will be visible to the viewer.
For typical hand-held devices the problem may become even more pronounced, as typically light with divergence of 7 degrees would illuminate about one pixel. Therefore extra measures and solutions are required to spread semi-directional illumination light over multiple pixels.